An excellent time to visit Thailand as the tropical weather is a lot less harsh and is actually quite pleasant. There's a lot of things to do in Thailand during the early months of the year which include: the Chiang Mai flower festival and Chinese New Year (celebrated in Bangkok's Chinatown).
More excellent weather in Thailand during these months as the time moves steadily closer to the rain season. An attraction to look out for during this time is the Thai New Year (Songkran), in which there are a plethora of celebrations all throughout Thailand.
Rain season is upon us during these months. Not an ideal time to visit, but you can still catch some excellent tropical weather at Hua Hin or Koh Samui.
September is Thailand wettest month so beachgoers are advised to avoid a trip during this time. A good bargain is always available during September if you decide to visit. October, on the other hand, has a lot drier weather and less humidity so it makes it an ideal time to visit the country.
The peak time to visit Thailand, the 'winter' months of November and December are the best time to visit Thailand due to the excellent weather.